Re:Fast as C but uses lots more memory (Score 1)

This is a incorrect. There are optimizations that you can perform in a tracing JIT which you simply cannot perform in a static compiler, even a static compiler with global optimizations on!

For instance, one of the optimizations we will be working on in the Spidermonkey tracer is escape analysis: if, during the course of a traced loop, a value which would normally be heap allocated goes out of scope, you know you can place that value on the heap. Because it's a tracing JIT, this optimization works across multiple methods.

Even standard compiler analyses such as common subexpression elimination and loop invariant hoisting can be much more effective in a tracing JIT than they can in a static compiler or even a per-method JIT: you know the side effects of everything that happens under the trace, and so you can know, across method boundaries, whether a traced loop is pure and whether loop counters are invariant or can be simplified.